Electric wave filter for thermionic valve amplifiers



w. s. PERCIVAL 2,226,739 ELECTRIC WAVE FILTER FOR THERMIONIQ VALVE AMPLIFIERS Filed Sept. 15,1938 2 Sheets- Sheet l INVENTOR mam-Ru L ATTORNEY Dec. 31, 1940. I qv 2,226,739

ELECTRIC WAVE FILTER FOR THERMIONIC VALVE AMPLIFIERS Filed Sept. -15, 19:58 2 Sheets-Sheet 2 /9. z fl INVENTOR W. S. PERCU/AL /W'l/-W ATTORNEY Patented Dec. 31, 1940 UNITED STATES PATENT OFFICE ELECTRIC WAVE FILTER FOR THERMIONIC VALVE AMPLIFIERS tion of Great Britain Application September 15, 1938, Serial No. 230,016 In Great Britain September 22, 1937 7 Claims The present invention relates to electric wave filters for thermionic valve amplifiers and the like such as are suitable for use in television systems, and is an improvement in the arrangements disclosed and claimed in my copending patent application, Ser. No. 126,317, filed Feb. 18, 1937, and issued as Patent No. 2,156,656 on May 2, 1939.

In the aforesaid application circuit arrangements forming electric wave filters suitable for use in television amplifiers are described in which a high value of the product of pass band and stage gain can be obtained. Circuits of low-pass type and of band-pass type are described, the low-pass type fil ters being in the form of filters which are matched at one end to prevent reflection and being provided with a mid-shunt termination at the other end with an additional ca pacity equal to the shunt capacity already existing, while the band-pass type circuits include a shunt inductance in addition to the shunt capacity, theinductance being provided to tune the additional shunt capacity to the mean frequency of the pass-band. v

In the case of filters of theband-pass type however, ordinary constant K type filters comprise a large number of elements and are somewhat diificult to adjust.

The object of the present invention is to provide band-pass type circuits in accordance with the teaching of the above mentioned application, which may be readily made up and adjusted.

According to the present invention, the filter is of the band-pass type andcomprises a plurality of prototype half-sections, the arrangement being such that at least one section of the filter includes no series tuned elements.

In accordance with one feature of the invention a filter may be provided in which part of the filter is low-pass and part band-pass, the two parts being substantially matched over the efiective range of the band-pass part.

In accordance with another feature of the invention a band-pass filter may be provided comprising one or more half-sections derived from constant K type half-sections in such a way that either series inductance elements or series capacity elements, and preferably the latter, are made to disappear.

In order that the method of carrying the invention into practice may be fully understood, the same will now be described with reference, by way of example, to the accompanying drawings in which:

Figure 1 shows a typical band-pass filter made up of constant K sections or hall-sections,

Figure 1a shows a filter equivalent to that of Figure 1, showing the constituent parts of the filter in expanded form,

Figure 2 shows a filter according to the prescircuit is effectively that represented in Figure 5,

Figure 9 shows a further form of filter in accordance with the present invention, and equivalent to that shown in Figure 6,

Figure 10 is a graph showing the response of a filter arranged as shown in Figure 6 or 9,

Figure 11 shows af urther form of filter provided according to the invention, and

Figure 12 is a graph showing the response of a further form of filter in accordance with the invention.

The arrangement shown in Figure 1, is a bandpass type circuit in accordance with the above mentioned application. This comprises a series branch comprising inductance i and condenser 2 in series and a shunt branch including condenser a and inductance b in parallel, and a further series branch including inductance c and condenser d in series with condenser e and inductance f in shunt.

The filter is terminated at one end by a resistance 5 having a value substantially equal to the characteristic impedance of the network and the inductance c and condenser it provide an unmatched termination as indicated in Figure 1a. For convenience in explaining the present invention the circuit of Figure 1 has been resolved, in Figure 1a, into its three component half-sections A, B and C respectively, each comprising an inductance such as I in series with a capacitance such as 2, and a shunt inductance such as 3, and a shunt capacitance such as 4, only the elements of half-section A and the terminating elements being referenced. The filter is matched at one end by terminating resistance 5 and has an unmatched termination D providing the initial shunt reactance at the other end, said termination com-prising capacitance 6 and inductance in shunt, the unmatched termination providing in association with the shunt branch of the preceding half-section of the filter an initial shun-t reactance equal to the full shunt reactance of a section of the filter.

It will be seen that the above described filter comprises as many series tuned branches as there are sections in the filter, and as in many cases, the tuning adjustments required are critical, serious limitations are placed on the use of the filter and. in addition a large number of elements may be required to make up a filter.

Thus according to one feature of the invention the number of series tuned arms required may be reduced by constructing a filter of lowpass and band-pass sections in combination. A suitable form of filter is shown in Figure 2. In this filter part E is a T type low-pass filter section comprising series inductances 8 and 9 and shunt capacity I0, and part F provides a bandpass half-section corresponding to C of Figure 1 and an unmatched termination corresponding to D of Figure 1a, the part F including series connected inductance H and capacity l2 and shunt connected inductance l3 and capacity 14. If the shunt capacities are not in the correct ratio this can be adjusted by the method of impedance transformation described in the textbook Transmission Networks and Wave Filters by T. E. Shea (Chapman 8: Hall, 1930) at page 326. This method is referred to more fully in the description with reference to Figure 11 of the drawings.

In most cases, for convenience, the inductances 9 and II of Figure 2 would be provided by the inductance of a single coil.

The method of designing a filter of the kind shown in Figure 2 will be readily understood from the following explanation:

For a filter made up of constant K type band-pass sections, in which the frequency limits of the pass-band are f1 and f2 respectively, the mean frequency is im, f and F2 respectively are the frequency limits of the working range of the filter and at which the characteristic impedance of the filter is R, R0 is the nominal impedance of the filter, the following basic relationships apply:

jhf2=f 1f 2=fm -1 1) and according as to whether R is the mid-series or mid-shunt characteristic impedance. I

A low-pass filter may be regarded as a special case of a band-pass filter where f1=f 1=0, f2 having a value in, the cut-off point for the filter. Thus for a low-pass filter where fm is the mean frequency of the bandpass half-section of the filter and and where ,f1 and f2 are the frequency limits of the pass band in the band-pass section, and .fm is the mean frequency thereof.

A very satisfactory arrangement is provided by the case in which the filter of Figure 2 is designed in such a manner that the working range of the band-pass portion coincides with the upper two thirds of the working range of the low-pass portion of the filter.

Assuming that the upper limit of the working range of a constant K low-pass section is given by the working range of the band-pass part will be from fi=% t ft= and the mean frequency fm thereof will be 0.288 lo.

Now from Equations 2 and 3 above It is thus possible to calculate the values of the elements in the band-pass part of the filter from the cut-off frequency of the low-pass section having the desired working range.

A characteristic response curve for a filter of the semi-band-pass type as shown in Figure 2 is shown by the curve I in Figure 3, wherein response in decibels is plotted vertically and frequency horizontally.

In this case the value of the various elements of the filter was as follows:

The frequencies and frequency parameter n for the filter are 10:10 mc./s. f1=1.06 mc./s. f2=7.72 mc./s. f 1=1.67 mc./s. f z=5 mc./s. fm=2.88 mc./s. n=2.31

It is of interest to notice the error in matching between the low-pass and the band-pass parts of the filter shown in Figure 2. For this purpose, the table below indicates the ratio R/Ro for each section:

R/R for R/Ra for Frequency low-pass band-pass section part tfo 98 87 96 1. 0 /ZfO 87 87 It Will thus been seen that the mis-match is only serious in the working range at low frequencies.

The average mis-match may bereduced by raising the impedance of the low-pass portion of the filter, say by Alternatively, the additional mid-shunt impedance at the open end may be varied to counteract the effect of the mis match on the amplitude response curve, for example, in the particular case given above it has been found advantageous to change-thev'alues of the elements I3 and I4 to 170 micro-henries and 19.6 micro-micro-farads respectively.

In the arrangement according to the invention shown in Figure 4, the two half-sections Band C of the filter shown in Figure 1a have been re placed by two m-derived half-sections arranged as indicated in column III of the table on page 316 of the aforesaid text book TransmissionNet works and Wave Filters. In the two said m: derived half-sections the series capacity elements have been eliminated, giving an arrangement of 1r-C0I1I1E0t9d inductances I5, I6 and I! with shunt condensers I8 and I9, the number of components being reduced by using one inductance or condenser to provide the elements of two sections or half-sections of the filter in well-knownmanner'. In this arrangement it will be seen that a series tuned circuit comprising elements I and 2 is still necessary for the constant K half-section and the arrangement is not very economical asfar as the number of elements is concerned.

It is preferred to eliminate series condensers as described rather than series inductances as the latter arrangement gives an improved product of stage gain and band width.

The filter arrangement of Figure 4 may be improved in two ways. Either the constant K section may be replaced by a low-pass section L of the same nominal impedance as in Figure 5, or some of the shunt capacity may be sacrificed =with a consequent lowering of the product of band width and stage gain and a mid-shunt terof the vr-COIIDGCIZGd inductances I5, I6 and I! of Figure 4 are excessive. Thus it is usually preferable to provide equivalent T-connected inductances 2e. El and 22 as shown in Figures Sand 6.

The arrangement of Figure 5 is similar to an ordinary inductance coupled band-pass circuit except that the whole of the resistance 5 is at one end. This renders it superior to the ordinary coupled band-pass circuit since the equivalent shunt resistance is higher for a given total capacity. In the same way a low-pass filter amplifier with a shunt capacity termination I9 at an open end as shown enables the same pass band to be obtained for a greater total shunt capacity than if both ends are terminated with a resistance. Moreover, the characteristic is flatter over the working range.

In a typical case, a filter in accordance with Figure 5 was made upof elements having the following values:

Resistance 5 2300 ohms.

Inductance 27 36.6 micro henries. Capacity 25 17.3 micro-micro-farads. Inductance 20 71.2 micro-henries. Inductance 22 223 micro-henries. Inductance 21 8.6 micro-henries. Capacity 19 20.75 micro-micro-farads.

The amplitude-frequency characteristic in this case is given by curve II in Figure 7.

Inductance 15 82.2 micro-henries. Inductance 16 212 5 micro-henries. Inductance l7 258 micro-henries.

Thus it will be readily seen that the T-connection of the inductances is to be preferred.

- In the particular case given, it was found that I the response curve might be made flatter as indicated at 11a in Figure 7 by slightly adjusting the values of the T-connected inductances 20, 2I and 22 as follows:

Inductances 20 71.9 micro-henries. Inductances 22 292 micro-henries. Inductances 21 7.8 micro-henries.

A typical filter amplifier forming a wave filter in accordance with Figure '5 is shown in Figure .8 of the drawings, corresponding elements in Figures 5 and 8 being indicated by the same reference numerals. The amplifier comprises two valves 23 amaze shown as of the screen grid type. The terminating resistance 5 of the filter is constituted by the anode load resistance of the valve .23, the shunt capacity 25 of the low pass section L of the filter is represented in dotted lines in Figure 8aand is actually the anode to cathode capacity of the valve 23., The condenser 26 is a coupling condenser and'is so large that the filter constantszare not affected by it. The inductances 2B ,ZIgand22 areincluded in the input. of valve 26 as shown, the inductance .2I being constituted by coupling between inductances 2B and 22 indicated by the line linking the two coils. The capacity I6 is shown dotted and is constituted by the grid-cathode or input capacity of the valve 26. The cathode-earth lead of each valve includes a biasing resistance 29a and 29 and associated by-pass condensers 28 and 30.

The filter arrangement of Figure 6 is a preferred alternative to the arrangement of Figure 5. The form of the half-sections used in derivingthe filter is that of the shunt derived halfsection shown in the bottom horizontal row of Figure 9 in the paperpresented before the Institution of Post Office Electrical Engineers on The Design and Construction of Electric Wave Filters, by R. J.. Halsey, the notation used in describing the arrangement of Figure 2 in the present case being due to Halsey and followed in the aforesaid Figure 9 of the paper. Using the form of half-section referred to, an arrangement simila-rfto that of Figure 6 of the present drawings is obtained'but having 1r-connected inductances I5 I6 and I'I as shown in Figure 9 of the present drawings instead of theT-connected inductances 25?, 2I and 22 as shown in Figure 6.

The methodof arriving at the form of filter shown in Figure 6 will be described in the first place with reference to Figure 9, which shows a filter similar to that shown in Figure 6 except for the different arrangement of inductances mentioned, parts in Figure 9 corresponding to thoseshown in Figure 6 being similarly numbered. Thus, referring to Figure 9, assuming that the Working pass-band or Working range of ill the filter is to be half the theoretical pass band, then again adopting Halseys notation, we have also f2 f1 =f m=f2f1 Thus eliminating f1 from Equation 4 and transposing we have i 1 f2 f2 j Likewise it can be shown that Thus, for a given working range, A and f2 can both be calculated. For convenience, as in the case of Figur 2, the theoretical Values of the elements of Figure 9 have been inserted in the figure, the Halsey notation being followed except that the symbol m has been used instead of ii m2 for f2 As already mentioned the values for the 1rconnected inductances thus obtained are excessive and it is preferred to transform to a T-connected arrangement as shown in Figure 6, using the method described in the aforesaid text book by Shea at page 93. The values of the T-connected inductances thus obtained are indicated in Figure 6, where In a typical case in a filter of the form shown in Figure 6, the values of the elements were as follows:

Resistance 2300 ohms.

Capacity 23 10.4 micro-microfarads. Inductance 20 I. 71.2 micro-henries. Inductance 22 223 micro-henries. Inductance 21 8.6 micro-henries. Capacity 19 20.75 micro-microfarads.

Inductance 20 69 micro-henries. Inductance 22 277 micro henries. Inductance 21 10.7 micro-henries. Capacity 19 18.2 micro-micro-farads.

In a case where 1r-connected inductances are .used as shown in Figure ,9, the values of these inductances would be Inductance 16 2125 micro-henries. Inductance 82.5 micro-henries. Inductance 17 258 micro-henries.

, It .will readily be understood that, as in the 'case'of the arrangement of Figure 5, inductance 2| of-Figure 6 may be produced by coupling between inductances 20 and 22.

The circuits of Figures 5, 6 and 9 are limited to a particular ratio between the shunt capacities, the ratio in the case of Figures 6 and 9 being 2:1, and it may be awkward, in practice, toprovide aflfilter with the requisite capacity ratio due to' the fact that these capacities in question are usually the inter-electrode capaci ties 01 valves used in the circuit. While one of the valve capacities might be suitably built out to the, required value with additional capacity it is more frequently preferred to insert a transformer having a suitable ratio of transformation between the two capacities to enhance the efiect of one of the capacities relatively to that of the other. In this way the product of pass band and stage gain is maintained at the maximum.

For example a transformer may be arranged between the inductances 22 and 2| of Figure 6 as shown at 3| in Figure 11.

In this case if the transformer 3| is ideal and has a voltage transformation ratio it may be shown following the reasoning in the aforesaid text-book by Shea at page 326, that the effective value of inductance 2| is given by dr g-1% andofcapacity l9 by I 200 1 where I Inductance 2O =L 4 L Inductance 21 (L 21,1 ?2

Inductance 22 L It may be shown that with the inductances arranged to produce the effect of a transformer as above described, under certain conditions, the inductance in either the right-hand limb or the left-hand limb of the T-section of the filter shown in Figure 6 may be made zero.

Thus it may be shown that if inductance 2| is to be eliminated.

In this case 1 for to be real, so that the ratio of capacity l9 to capacity 23 must be greater than 2:1.

Similarly if coil 20 is to be eliminated, since L must equal L it may be shown that and for a real solution of this equation, must be greater than 1 so that the aforesaid capacity ratio must be less 2:1.

Figure 12 shows, by way of example, the characteristic curve IV of a filter in which coil 20 was eliminated, the remaining elements having the following values:

Resistance 5 1500 ohms.

Capacity 23 18.3 micro-micro-farads. Inductance 22 82.5 micro-henries. Inductance 21 82.5 micro-henries. Capacity 19 12.2 micro-micro-farads.

The frequeniies and frequency parameters n and A for the filter having the characteristic IV of Figure 12 is as follows: jm=4.1 mc./s., f1=2.1 mc./s., f2=7.9 mc./s., f1 =2.9 mc./s., f2 =5.8 mc./s., =1.41, A=2.

By reducing the inductance 22 by 12% it was found that the response curve of the filter was modified substantially as indicated by the dotted line 1V at the right-hand end of the curve IV.

However, for some capacity ratios, the required mean frequency to permit such elimination may be unsuitable, though it so happens that when, as in the case of the illustrative example given above, the capacity ratio is 1.5:1, the left hand inductance can be caused to disappear if the larger shunt capacity is placed in parallel with the terminating resistance and a mean frequency of about 1.5 times the pass-band is selected.

It will be understood that while only an arrangement of T-connected inductances has been shown for producing the desired transformer effect, an equivalent arrangement of 1r-C0-1'1I1ECt8d inductances could be used.

What I claim is:

1. An electric wave filter of the band-pass type comprising a plurality of prototype half-sections, each half-section comprising a series and a shunt reactance, one end of the filter being terminated by an impedance substantially equal to the characteristic impedance of the network, in which for the purpose of maintaining the impedance at the unterminated end of the filter substantially constant over the pass band, the value of the initial shunt reactance is made of the order of that of a full shunt reactance, the arrangement being such that the series portion of at least one section is composed solely of reactance of a single sign.

2. An electric wave filter according to claim 1 in which part of the filter is low-pass and part band-pass, the two sections of the filter being substantially matched with respect to their impedance characteristics over the efiective range of the band-pass part.

3. An electric wave filter in accordance with claim 1 comprising a low-pass section to one end of which the terminating impedance for the filter is connected and to the other end of which a band-pass half-section is connected, said bandpass half-section having a shunt reactance which forms the initial shunt reactance of the filter and is made equal to the shunt reactance of a full band-pass section.

4. An electric wave filter according to claim 1 comprising a low-pass section to one end of which the terminating impedance for the filter is connected and to the other end of which a bandpass half-section is connected, said band-pass half-section having a shunt reactance which forms the initial shunt reactance of the filter and is made equal to the shunt reactance of a full band-pass section the arrangement being such that the working range of the band pass portion of the filter corresponds with the upper two-thirds of the working range of the low-pass section thereof.

5. An electric wave filter in accordance with claim 1 including a band-pass section the series portion of which is composed solely of reactance of a single sign, the shunt reactance at one end of the band-pass section constituting the shunt reactance of the filter and being equal to the shunt reactance of a full section.

6. An electric wave filter in accordance with claim 1 including a band-pass section the series portion of which is composed solely of reactance of a single sign, and wherein the filter includes in association with said band-pass section a lowpass half-section to one end of which the terminating impedance for the filter is connected.

7. An electric wave filter according to claim 1 in which part of the filter is low-pass and part band-pass, the two sectiom of the filter being substantially matched with respect to their impedance characteristics over the efiective range of the band-pass part, saidband-pass section of the filter comprising an arrangement of 3 T-connected inductances.

WILLIAM SPENCER PERCIVAL. 

